Polyurethane coatings are well known and have gained commercial acceptance as protective and decorative coatings for metal, wood, concrete, foam, and plastics in the aircraft, construction, product-finishing, textile and maintenance/architectural coatings markets. The basic raw materials used to prepare these coatings generally comprise as essential components (a) an aliphatic or aromatic di-or-polyisocyanate and (b) a co-reactant or curative component bearing active hydrogen containing groups, i.e., hydroxyl or amine groups, or latent active hydrogen containing groups, e.g., oxazolidines or ketimines. For typical two-package coating systems, the co-reactant is usually a hydroxyl group containing oligomer chosen from the general classes of polyesters, polyethers, alkyd resins and acrylics. The co-reactant component is generally the vehicle for pigment (grinding) and may also contain other additives such as catalysts, plasticizers, bitumenous extenders, suspending agents, anti-skinning agents, surfactants, and rheological modifiers.
Both the isocyanate-containing component and the active hydrogen-containing co-reactant usually contain volatile organic solvents whose primary function is to lower viscosity thereby providing a consistency suitable for spray application with conventional air, airless and electrostatic spray equipment.
A growing emphasis on compliance with government environmental and health hazard regulations that limit both the type and amount of volatile organic compounds (VOC) has prompted coating manufacturers and end users to evaluate new coating technologies.
Prior art high solids and solventless polyurethane coatings have been developed which comply with solvent emission regulations. As used herein, a solventless polyurethane coating is one in which substantially all of the constituents remain in the applied coating.
The first solventless, urethane coatings were the "one shot" systems, so named because no prereaction of components is involved. Typical "one shot" systems consist of a pure isocyanate component, usually 4,4'-diphenylmethane diisocyanate (MDI), and a curative component comprised of a blend of active hydrogen containing co-reactants, for example polyether or polyester polyols and lower MW glycol, with fillers and catalyst. The components are usually combined at volumetric mix ratios of 1:1 to 4:1. Although coating systems of this type are sprayable without the use of a solvent, there are some disadvantages. One shot systems are moisture sensitive because they contain a very high percentage of unreacted diisocyanate, usually 26-31% by weight, and cannot be sprayed under humid conditions without blowing or foaming. Another major disadvantage of one shot spray systems is reflected in the low physical properties of the polyurethanes they produce. Important physical properties such as tensile and tear strength and abrasion resistance are inferior to those obtained by solvent containing systems.
Prior art two package, solventless polyurethane coating systems with superior physical properties have also been developed. These spray systems are generally based on an isocyanate-containing prepolymer component combined with a curative component comprised of a viscous, sometimes solid, polyamine whose viscosity has been reduced by addition of a non-volatile diluent or whose volume has been adjusted with "polyol" so that it can be combined with the isocyanate component at predetermined volumetric mix ratios.
U.S. Pat. No. 4,195,148 and U.S. Pat. No. 4,234,445 disclose solventless polyurethane coatings which use a non-reactive, non-volatile lactone viscosity modifier to dissolve viscous or solid prepolymer and curative components to reduce viscosity of the systems so that they may be applied using airless, plural-component spray equipment.
U.S. Pat. No. 4,267,299 also discloses sprayable, solventless polyurethane compositions which combine an isocyanate terminated prepolymer component (Part A) with a curative component (Part B). The curative component is a blend of highly reactive polyamine with a slower reacting, higher molecular weight (MW) polyol. Part B includes enough highly reactive polyamine to react with substantially all of the isocyanate groups in the Part A within 2 to 5 minutes. The higher MW polyol is present in relatively small amounts and functions as a "reactive filler" or bulking agent which allows the Part A and Part B to be combined in ratios of from 2:1 to 4:1.
The polyurethane produced from preferred embodiments of prepolymer-based, prior-art inventions (U.S. Pat. No. 4,195,148, U.S. Pat. No. 4,234,445 and U.S. Pat. No. 4,267,299) have superior physical properties such as tensile strength, tear strength, and abrasion resistance compared to one-shot, solventless coating systems. Moreover, such prepolymer-based solventless coating systems are generally less sensitive to moisture than one-shot solventless systems because pre-reaction of the isocyanate with polyol to form the prepolymer component results in less reactive isocyanate, typically 3 to 12% by weight.
There are some disadvantages associated with prior art, prepolymer based solventless polyurethane coatings. For optimum mixing, a ratio of 1:1 is desirable; preferred embodiments of the prior art have volumetric mix ratios of 3:1. A major disadvantage of preferred embodiments of prior-art, prepolymer-based, solventless, polyurethane coatings is that the components must be heated to 160.degree. F.-200.degree. F. to reduce viscosities for spray application. Heating, maintaining and applying components at 160.degree.-200.degree. F. requires additional equipment such as drum heaters to warm viscous materials to a pumpable viscosity, electrical induction heaters to further raise component temperatures and reduce viscosity, and the use of heated hoses to maintain temperature until the components enter the spray gun. Prior-art systems have fast gel times, typically 0.5 to 3 minutes at application temperatures of 160.degree.- 200.degree. F., and must be applied with an internal mix, plural-component spray gun such as the Binks 43-P. Internal mix spray guns of this type are connected to three spray hoses; one for each component and one for solvent flushing. The hoses increase the weight of the spray gun, make the gun awkward to use, and severely restrict the movement of the applicator in confined areas such as manholes or covered hopper cars. Accordingly, a solventless, polyurethane coating composition is needed which combines the ease of application at ambient temperatures and superior physical properties.